SQL Stored Procedure - Understanding SQL Statement - sql

Question

New to stored procedures. Can anyone explain the following SQL sample which appears at the start of a stored procedure?
Begin/End - Encloses a series of SQL statements so that a group of SQL statements can be executed
SET NOCOUNT ON - the count (indicating the number of rows affected by a SQL statement) is not returned.
DECLARE - setting local variables
While - loops round
With - unsure
Update batch - unsure
SET #Rowcount = ##ROWCOUNT; - unsure
BEGIN
SET NOCOUNT ON;
--UPDATE, done in batches to minimise locking
DECLARE #Batch INT= 100;
DECLARE #Rowcount INT= #Batch;
WHILE #Rowcount > 0
BEGIN
WITH t
AS (
SELECT [OrganisationID],
[PropertyID],
[QuestionID],
[BaseAnsweredQuestionID]
FROM dbo.Unioned_Table
WHERE organisationid = 1),
s
AS (
SELECT [OrganisationID],
[PropertyID],
[QuestionID],
[BaseAnsweredQuestionID]
FROM dbo.table
WHERE organisationid = 1),
batch
AS (
SELECT TOP (#Batch) T.*,
s.BaseAnsweredQuestionID NewBaseAnsweredQuestionID
FROM T
INNER JOIN s ON t.organisationid = s.organisationid
AND t.PropertyID = s.PropertyID
AND t.QuestionID = s.QuestionID
WHERE t.BaseAnsweredQuestionID <> s.BaseAnsweredQuestionID)
UPDATE batch
SET
BaseAnsweredQuestionID = NewBaseAnsweredQuestionID
SET #Rowcount = ##ROWCOUNT;
END;

Answer 1

The clue is in the comment --UPDATE, done in batches to minimise locking.
The intent is to update dbo.table's column BaseAnsweredQuestionID with the equivalent column from dbo.Unioned_Table, in batches of 100. The comment suggests the batching logic is necessary to prevent locking.
In detail:
DECLARE #Batch INT= 100; sets the batch size.
DECLARE #Rowcount INT= #Batch; initializes the loop.
WHILE #Rowcount > 0 starts the loop. #Rowcount will become zero when the update statement affects no rows (see below).
with a as () is a common table expression (commonly abbreviated to CTE) - it creates a temporary result set which you can effectively treat as a table. The next few queries define CTEs t, s and batch.
CTE batch contains just 100 rows by using the SELECT TOP (#Batch) term - it selects a random 100 rows from the two other CTEs.
The next statement:
UPDATE batch
SET BaseAnsweredQuestionID = NewBaseAnsweredQuestionID
SET #Rowcount = ##ROWCOUNT
updates the 100 rows in the batch CTE (which in turn is a join on two other CTEs), and populates the loop variable #Rowcount with the number of rows affected by the update statement (##ROWCOUNT). If there are no matching rows, ##ROWCOUNT becomes zero, and thus the loop ends.